The inventive concepts described herein relate to semiconductor memory devices, and more particularly, relate to impedance calibration of memory devices.
Semiconductor memories are widely used components of digital logic system design, such as computers and microprocessor-based applications ranging from satellites to consumer electronics. Therefore, advances in the fabrication of semiconductor memories including process enhancements and technology developments through the scaling for higher densities and faster speeds help establish performance standards for other digital logic families.
Semiconductor memory devices may be characterized as volatile random access memories (RAMs), or non-volatile memory devices. In RAMs, the logic information is stored either by setting up the logic state of a bistable flip-flop such as in a static random access memory (SRAM), or through the charging of a capacitor as in a dynamic random access memory (DRAM). In either case, the data are stored and can be read out as long as the power is applied, and are lost when the power is turned off; hence, they are called volatile memories.
Non-volatile memories, such as Mask Read-Only Memory (MROM), Programmable Read-Only Memory (PROM), Erasable Programmable Read-Only Memory (EPROM), and Electrically Erasable Programmable Read-Only Memory (EEPROM), are capable of storing the data, even with the power turned off. The non-volatile memory data storage mode may be permanent or reprogrammable, depending upon the fabrication technology used. Non-volatile memories are used for program and microcode storage in a wide variety of applications in the computer, avionics, telecommunications, and consumer electronics industries. A combination of single-chip volatile as well as non-volatile memory storage modes is also available in devices such as non-volatile SRAM (nvSRAM) for use in systems that require fast, programmable non-volatile memory. In addition, dozens of special memory architectures have evolved which contain some additional logic circuitry to optimize their performance for application-specific tasks.
In non-volatile memories, however, MROM, PROM, and EPROM are not free to be erased and written to by a system itself, so that it is not easy for general users to update stored contents. On the other hand, EEPROM is capable of being electrically erased or written. Application of the EEPROM is widened to an auxiliary memory or to system programming where continuous updates are needed (flash EEPROM).
Many memory devices use impedance calibration (often referred to as “ZQ calibration”) to improve the performance thereof. Impedance calibration may be used to calibrate the input/output circuits of the memory device to account for variations in the system environment of temperature, voltage, component drift, etc.